1. Technical field
The present invention relates to a printed wiring board and an electronic device using the same, such as a semiconductor device and a multi chip module.
2. Background Art
In recent years, electronics devices such as notebook-sized personal computers, digital cameras, video cameras, and personal digital assistants (PDA) including mobile phones have become increasingly smaller, lighter, and more sophisticated. As a result, there is a need for higher density in printed wiring boards, which support and fix electronic components constituting the above devices and form a circuit, semiconductor devices such as BGA (Ball Grid Array)/CSP (Chip Scale Package) using printed wiring boards, or multi chip modules.
In order to deal with this densification trend, new wiring boards such as build-up substrates and glass substrates have been suggested and already put into practice.
As one example of such a wiring board, FIG. 9 shows a schematic cross-sectional view of a printed wiring board forming two wiring layers on one side of an insulating substrate.
Basically, a base metal layer 2 and a metal conductor 3 are formed on the surface of an insulating substrate 1, on the top of which an insulating resin layer 4 (interlayer insulating layer) is formed. Further, on top of this a base metal layer 5 and a metal conductor 6 are formed, and a protective insulating resin layer 7 is formed thereon.
In FIG. 9 shows a case wherein a printed wiring board has base metal layers 2 and 5, but these layers may not be formed depending on production methods.
One example of these methods for producing a printed wiring board will be explained based on FIG. 12. First, as shown in FIG. 12(a), a base metal layer 2 is formed on the surface of an insulating substrate 1. The base metal layer 2 is formed for imparting continuity to conduct electroplating or the like and for enhancing adhesiveness between the insulating substrate 1 and the metal conductor 3. As a base metal material, a metal such as chrome, which has good adhesiveness to the insulating substrate and insulating resin layer, is preferred, although the base metal material is not particularly limited thereto.
Examples of formation methods include vacuum evaporation, spattering, and electroless deposition, any of which may be employed. Further, when the metal conductor 3 is formed by electroless deposition, the base metal layer 2 is not necessary.
Next, in FIG. 12(b), a resist 9 is formed by spin coat method, etc., and the exposure and development is carried out using a pattern mask so that the resist 9 having a negative pattern of a desired circuit pattern is obtained.
Next, in FIG. 12(c), using the base metal layer 2 as an electrode, electroplating enables the metal conductor 3 to grow on portions where the resist 9 is not formed, thereby obtaining a desired circuit. Further, in FIG. 12(d), the resist 9 is removed with a chemical, or by other means
Then, in FIG. 12(e), the base metal layer 2 is removed with a chemical, etc., except for a portion of the base metal layer 2 on which the metal conductor 3 is formed.
In FIG. 12(f), an insulating resin such as polyimide is applied by spin coat method, etc. to form an insulating resin layer 4 (interlayer insulating layer). Then, a first layer of wiring is completed.
Subsequently, in the processes (g) to (l) of FIG. 12, a second layer of wiring is formed in the same manner as in the processes (a) to (f). Moreover, when it is desired to increase wiring layers, the processes (g) to (l) may be repeated. Although not shown in FIGS. 9 and 12, a via hole or a through hole may be formed to establish the continuity between the first and second layers. The continuity between wiring layers leads to the completion of a printed wiring board.
With respect to the printed wiring board fabricated by the above method, when spaces 11 between metal conductors (wiring) are made smaller (fine pitch) for densification, it is found that there is the fear that the following problems will arise. FIGS. 10 and 11 are enlarged views of Portion A of a printed wiring board of FIG. 9, and the above problems are described in detail by way of FIGS. 10 and 11.
When the printed wiring board fabricated by the method of FIG. 12 has smaller spaces 11 between metal conductors (wiring) as shown in FIG. 10, it becomes difficult to remove the base metal layers 2 and 5 in the processes (e) and (k) of FIG. 12. These processes involve removing the unnecessary portion of the base metal layer 2 except for the circuit on the insulating substrate 1, and removing the unnecessary portion of the base metal layer 5 except for the circuit on the insulating resin layer 4 (interlayer insulating layer). Therefore, the base metal layers remain between wirings (remaining base metal layer 12).
This remaining base metal layer 12 reduces the insulation resistance value of spaces 11 between metal conductors (wiring). Further, when the base metal layers 2 and 5 are formed by spattering, etc., the base metal layers are embedded in the insulating substrate 1 and the insulating resin layer 4 (interlayer insulating layer) and thus the insulation resistance value tends to be lower.
Moreover, even if the base metal layers 2 and 5 are completely removed, chemicals such as etchants remain in spaces 11 between metal conductors (wiring) and become ionic impurities 13. As a result, the insulation resistance values of the spaces 11 between metal conductors (wiring) are lowered. This is a first major problem.
In addition, when voltage is applied to the spaces 11 between metal conductors (wiring) as shown in FIGS. 9 and 11 in a high temperature and in a highly humid environment, the base metal layers 2 and 5, and the metal conductors 3 and 6, are melted and ionized. The ions migrate on an interface portion between the insulating substrate 1 and the insulating resin layer 4 (interlayer insulating layer) or on an interface portion between the insulating resin layer 4 (interlayer insulating layer) and the protective insulating layer 7 (ion migration 15). As a result, deterioration phenomena such as electrical leakages or short circuits between wirings occur. This is a second major problem.
Incidentally, as a method for improving the insulation property between wirings, JP Patent Publication (Unexamined Application) No. 2000-183468suggests the formation of projecting ribs or recessed grooves on a joint resin surface.
However, the suggestion of this application aims at improving the insulation property of a resin molded wiring board (mold wiring board), which is prepared by insert-molding a metal frame with a resin. The wiring board disclosed in the above publication completely differs from the high-density printed wiring board that is covered by the present invention and manufactured by additive process, semi-additive process, and subtractive process in terms of production process, wiring density, etc. Thus, the wiring board of the above publication does not suffer any of the above problems because of its production process.
An object of the present invention is to solve the above first and second problems and provide a printed wiring board that has high insulation resistance value between metal conductors (wiring) and is unlikely to cause failures such as electrical leakages or short circuits due to ion migration or the like.
A further object of the present invention is to provide an electronic device using the above printed wiring board, such as a semiconductor device or a multi chip module.